1. Field of the Invention
The present invention relates to a two-stage balancer for a multi-lamp backlight, and more particularly to a two-stage balancer for providing much better current balance between parallel circuit loops to maintain uniform brightness of the multi-lamp backlight.
2. Description of Prior Art
In recent years, TFT-LCD has been widely used in different applications with progress of the TFT-LCD display technology, and more particularly in desktop personal computers and multi-media TVs. In order to achieve high-brightness performance for the large-size TFT-LCD applications, the amount of lamps required for a multi-lamp backlight is large. The amount of current flowed through the lamps in parallel is influenced by the impedance between the lamps. Hence, relative brightness of the lamp driven with smaller current is darker; on the contrary, relative brightness of the lamp driven with larger current is lighter to influence uniform brightness of the multi-lamp backlight. Furthermore, the unbalanced current flowed through the lamps will reduce the lifetime of the lamps and the multi-lamp backlight. Hence, it is important to balance the amount of current flowed through the lamps.
U.S. Pat. No. 7,242,147 disclosed a current sharing scheme for a multiple CCF lamp operation. The current sharing scheme includes a plurality of balancing transformers to facilitate current sharing in a multi-lamp backlight system. Reference is made to FIG. 1 which is a circuit block diagram of a prior art current sharing scheme. The current sharing scheme includes a first transformer group 706, a second transformer group 710, and a lighting apparatus group 708. The first transformer group 706 has a plurality of first balancing transformers 706_1˜706—k, the second transformer group 710 has a plurality of second balancing transformers 710_1˜710—k, and the lighting apparatus group 708 has a plurality of lighting apparatuses 708_1˜708—k. The first transformer group 706 is electrically connected to the second transformer group 710 for forming a ring connection. The lighting apparatuses 708_1˜708—k are electrically connected between the first balancing transformers 706_1˜706—k and the second balancing transformers 710_1˜710—k, respectively. Each of the first balancing transformers 706_1˜706—k has a primary winding and a secondary winding, and each of the second balancing transformers 710_1˜710—k has a primary winding and a secondary winding. The respective secondary windings of the first balancing transformers 706_1˜706—k are in series coupled together in a closed loop. Also, the respectively secondary windings of the second balancing transformers 710_1˜710—k are in series coupled together in a closed loop. Each of the primary windings of the first balancing transformers 706_1˜706—k is electrically connected to a first terminal of the corresponding lighting apparatuses 708_1˜708—k. Also, each of the primary windings of the second balancing transformers 710_1˜710—k is electrically connected to a second terminal of the corresponding lighting apparatuses 708_1˜708—k. 
However, each of the lighting apparatuses 708_1˜708—k is connected in series between one first balancing transformer 706_1˜706—k and one second balancing transformer 710_1˜710—k. Hence, distortions on the current waveform of the lighting apparatuses 708_1˜708—k are largely caused when any one of the first balancing transformers 706_1˜706—k or the second balancing transformers 710_1˜710—k is unbalanced. Because each of the lighting apparatuses 708_1˜708—k needs to match one first balancing transformers 706_1˜706—k and one second balancing transformers 710_1˜710—k, the use amount of magnetic components is large to make a higher cost.
Accordingly, a two-stage balancer for a multi-lamp backlight is provided to solve the above-mentioned disadvantages.